In the SITRANS TO500 multi-channel temperature transmitter, light is generated in the wavelength of 1500 to 1600 nm and output to the measurement lance by means of a continuously tunable laser light. Fiber Bragg gratings (FBG) are mounted at defined measurement points on the measurement lances. Each fiber Bragg grating reflects light of a defined wavelength. The wavelength reflected by the fiber Bragg grating varies as a function of temperature. The reflection at the fiber Bragg gratings is thus a measure of the temperature at the respective measurement point. A maximum of 20 to 48 fiber Bragg gratings per channel can be evaluated, depending on the temperature range.

A gas cell with fixed absorption line serves as a reference in the SITRANS TO500 and the wavelength determination is continuously adjusted by it.